1. Field of the Invention
The present invention relates to an automatic transmission primarily intended for motor vehicle use, and more particularly, to a method of controlling the partial frictional engagement or lock-up between a vehicle engine and a torque converter in a transmission that is controlled electronically and hydraulically.
2. Description of Related Art
A conventional transmission includes a hydrodynamic torque converter to transfer engine torque from the engine crankshaft to a rotatable input member of the transmission through fluid-flow forces. Although the idea of locking up the torque converter has been around for many years, few transmissions incorporated this feature before the fuel economy crunch of the '70s, because the fuel economy benefit of eliminating torque converter slip was not worth the driveability penalty that invariably resulted from eliminating the torque converter's damping effect. Until recently, all torque converter lock-up was of the full lock-up variety, e.g., the lock-up clutch would fully engage and prevent any slip. Engine torsional vibrations would mostly be absorbed in damper springs located between the lock-up clutch and the turbine hub (transmission input). The lower limit for engine r.p.m. depended on damper rate, number of cylinders, etc.; below this limit, high frequency vibrations ("Torsionals") made lock-up operation objectionable. In some cases, however, lower frequency disturbances, e.g., surge or bucking, raised this limit.
An alternative to the above is partial lock-up, a.k.a. controlled slippage of the lock-up clutch, which is disclosed in U.S. Pat. No. 4,468,988, issued Sep. 4, 1984, to Hiramatsu. No damper is necessary with this approach. Lock-up clutch capacity is modulated to control lock-up clutch slip at some desired value, perhaps 50 r.p.m. The engine's torsionals go to its own inertia, resulting in an engine speed variation of perhaps plus or minus 30 r.p.m., so the clutch slips continuously. Thus, the input torque to the transmission equals clutch capacity.
Currently, a method has been developed for determining and controlling the lock-up of a torque converter. An example of such a method is disclosed in copending patent application, U.S. Ser. No. 188,603, filed Apr. 29, 1988, entitled "Method of Determining and Controlling the Lock-Up of a Torque Converter in an Electronic Automatic Transmission System", which is hereby incorporated by reference. The method determines whether the torque converter is fully locked or unlocked. The method also checks conditions on whether full lock-up operation will be allowed. If the conditions do not allow full lock-up operation, a transmission controller solves equations for partial lock-up for actuating valves in the hydraulic system for partially locking up the torque converter.
3. Objects of the Present Invention
It is one of the principle objects of the present invention to provide a method of determining and controlling a lock-up mode of operation for the torque converter to reduce the rotational speed difference between the torque converter and the vehicle engine.
It is another object of the present invention to eliminate or reduce partial lock-up surge.